Fluorine-containing polymers have low surface energy and outstandingly high oxidation resistance and chemical resistance and other properties which are different from those of hydrocarbon-type polymers. Thus, these properties are arousing much research and development interest in fluorine-containing polymers.
On the other hand, extensive work has been done in recent years on ion exchange membrane for use as diaphragms in the electrolysis of alkali metal salts. These ion exchange membranes are required to have satisfactory resistance to alkali metal hydroxides kept at high temperatures and present in high concentrations and nascent-state halogen gases and oxygen gas, as well as a very high level of performance. To meet these requirements, ion exchange membranes composed of a sulfo-containing perfluorocarbon polymer were suggested. These ion exchange membranes are manufactured by E. I. du Pont de Nemours & Co. and sold under the trademark NAFION. Ion exchange membranes of perfluorocarbon polymers having sulfo groups as sole ion exchange groups have the defect that when they are used as electrolytic diaphragms in the electrolysis of alkali metal salts, they suffer from a low current efficiency. It is known that this defect can be effectively remedied by converting the sulfo groups partly to carboxyl groups. Japanese Laid-Open Patent No. 24177/77 discloses a process for producing an ion exchange membrane of a fluorocarbon polymer having both sulfo groups and carboxyl groups. The gist of this process is to convert sulfonyl halide groups to carboxyl groups through sulfino groups by reducing them with suitable reducing agents. Since, however, the sulfino groups are thermally and chemically unstable, the resulting ion exchange membrane gains a drastic increase in electric resistance. Moreover, it is sometimes necessary to perform an oxidation treatment in order to convert the unreacted sulfino groups to sulfo groups.
The present inventors have now found a novel process for converting sulfonyl halide groups bonded to the carbon atoms of a fluorocarbon polymer into carboxyl groups easily and efficiently. This process has made it possible to convert some or substantially all of the sulfonyl halide groups bonded to the carbon atoms of the fluorocarbon polymer into carboxyl groups.